TOMS

The Total Ozone Mapping Spectrophotometer (TOMS) was designed to produce accurate global estimates of total column ozone. It can also detect SO2 (gas), H2SO4 (sulfate) aerosols in the stratosphere, and UV absorbing aerosols (smoke, dust) over land and ocean. TOMS makes 35 measurements every 8 seconds, each covering 50-200 kilometers wide on the ground. Close to 200,000 daily measurements cover almost every spot on the Earth except for areas near the poles. These data make it possible to observe a variety of Earth events including forest fires, dust storms and biomass burning. NASA scientists have used TOMS data to chart the spread of smoke from large fire outbreaks, such as those in Western Brazil during August, 1998. (Image by NASA Goddard Space Flight Center TOMS project)           TOMS instruments have been carried on four satellites: 1. Nimbus-7
The Nimbus 7 satellite provided data for 14.5 years, from 1979-1993. The orbit was near-polar and sun-synchronous (13.8 sun-synchronous orbits per day with a near-noon equator crossing time). This orbit produced coverage of most of the Earth's surface on a daily basis. Nimbus 7/TOMS measured the amount of backscattered UV radiance in six 1 nm wide wavelength bands (313, 318, 331, 340, 360, and 380 nm).
2. Meteor-3
This satellite was in operation from August 1991 to December 1994. It was not sun-synchronous; its orbit processed from the sunrise terminator to the sunset terminator ever 106 days. The data from this TOMS is only comparable to Nimbus 7/TOMS and EP/TOMS when the orbit of Meteor 3 is close to the near-noon orbit (near-noon equator crossing time).
3. ADEOS (Japanese satellite)
ADEOS TOMS was launched on August 17, 1996 and provided data until June 29, 1997. Its orbit is higher than EP-TOMS, with a spatial resolution similar to Nimbus 7/TOMS.
4. Earth Probe TOMS
NASA's Earth-Probe satellite was launched on July 2, 1996 to provide supplemental measurements, but was boosted to a higher orbit to replace the failed ADEOS. Earth Probe continues to provide near real-time data. Its orbit is sun-synchronous and lower altitude than the previous TOMS platforms.
UV Absorbing Aerosols Data obtained from the TOMS instruments is used to obtain estimates of the quantities of aerosols in the troposphere that absorb ultra-violet light. In particular, desert dust and smoke from fires absorb at the UV wavelengths used by TOMS. Smoke detected by TOMS comes from a variety of ground based sources, such as biomass burning whether naturally occuring or caused by the agriculture, oil industy fires or industrial smoke. TOMS detects smoke particles regardless of source or season and is reliable over land or water. Data from all four NASA TOMS instruments have been used to produce information about the optical depth of these aerosols, although the wavelengths employed were slightly different. The TOMS aboard the Nimbus-7 & Meteor-3 satellites measured UV absorbing aerosols using upwelling radiances at 340nm and 380nm. The TOMS aboard the ADEOS and Earth Probe measured aerosols using the 331 and 360 nm wavelength channels. The TOMS aerosol optical depth record covers the periods from January 1979 to April 1993 (Nimbus7-TOMS observations), and from July 1996 to the present (Earth Probe TOMS measurements). The UV aerosol detection method is fundamentally different from aerosol measurements at visible and near IR wavelengths due to the existence of a strong Rayleigh scattering signature at the shorter wavelengths. In addition, the ground reflectivity is much lower and less variable in the UV compared with that at the longer wavelengths. As a result it is possible to image the aerosol clouds over land with TOMS, while aerosol retrievals from other instruments (such as the OCTS and POLDER) are limited to the oceans where the surface reflectivity is more predictable.

How TOMS data are used to detect aerosols

Data from TOMS can be used to detect the presence of both UV absorbing aerosols and nonabsorbing aerosols. The technique uses the ratio of the upwelling radiance (or spectral contrast) between the 340 nm and 380 nm channels (I340/I380). UV absorbing aerosols include smoke produced by biomass burning, black carbon from urban and industrial activities, agricultural dust, mineral dust coming from arid and semi-arid regions (desert dust) volcanic aerosols and ash. Carbonaceous aerosols generated by biomass combustion consist of a mixture of material with varying radiative properties; the absorbing fraction will contain elemental or graphitic carbon. Nonabsorbing aerosols are primarily sulfate (H2SO4) aerosols. UV spectral contrast is useful over both land and water because the UV reflectivity of these surfaces is low and nearly constant, unlike for the visible wavelengths. UV reflectivity of snow and ice, however, is high, therefore TOMS data is not as useful for detecting aerosols at high latitudes when snow and ice coverage produces high background surface reflection in the UV range. Gaseous absorption of UV is weak at the 340, 360 and 380 nm wavelengths. Backscattered radiation at these wavelengths is primarily controlled by Rayleigh (molecular) scattering, surface reflection (Earth's surface), and scattering from aerosols and clouds (Mie scattering). The inclusion of these wavelengths in the TOMS instrument provided a means for detecting the presence of aerosols. In a clear molecular atmosphere (no aerosols and clouds), molecular (Rayleigh) scattering at a given wavelength will scale inversely with l4. This mathematical relationship causes up to a 50% difference in the backscattered UV radiance between 340 and 380 nm in a pure atmosphere (ie. a strong spectral contrast). Mie scattering can also make reflectivity (R) spectrally dependent. UV absorbing aerosols cause R (reflectivity) to increase with wavelength (e.g. R380 > R340). For this reason, the presence of aerosols and clouds adds a radiance component that is weakly wavelength dependent; they reduce the I340/I380 spectral contrast that would be observed due to the molecular atmosphere alone. In other words, the Mie scattering caused by the aerosols or clouds reduces the spectral contrast expected due to Rayleigh scattering alone. The detection of aerosols from TOMS data involves a quantity called a residue. The N-value residue at l340 is defined as: DNl = -100{log10[I340/I380)meas] - log10[I340/I380)calc} where Imeas = the backscattered radiance at that wavelength measured by the TOMS and Icalc is the model calculated radiance assuming an atmosphere of Rayleigh scatterers (pure molecular atmosphere) bounded by a Lambertian surface (which necessitates exclusion of data affected by sea glint, snow or ice). The model employed is a modified version of Dave's LER model, constructed to give nearly zero residue in the presence of clouds. When UV absorbing aerosols are present in the atmosphere, the spectral contrast (I340/I380) is smaller than predicted by the LER model, and positive residues are produced by the equation above. Nonabsorbing aerosols produce greater spectral contrast, and thus result in negative residues. DN340 values scale almost linearly with single scatter albedo, (thus optical depth) and with altitude. UV absorbing aerosols in the boundary layer of the troposphere are not readily measured by TOMS because aerosol absorption at one height in the atmosphere affects molecular scattering below the aerosol layer, and the underlying Rayleigh scattering produces only a small signal. Starting at altitudes of at least 1 km, absorbing aerosols become readily detected by the residue technique. In the middle latitudes, most of aerosol transport occurs between 3-5 km altitude (or higher in the case of volcanic ash) thus middle latitude aerosols are well detected.

TOMS Aerosol Index

TOMS aerosol data are given in units called the aerosol index. The aerosol index (AI) is defined as the difference between the observations and model calculations from a pure molecular atmosphere with the same surface reflectivity and measurement conditions. The Index can be interpreted in terms of optical depth if the index of refraction, particle size distribution, and the height of the aerosol layer are known from other measurements. Below is an example of the aerosol index obtained for July 24, 2002. Saharan dust can be seen above the continent and moving westward into the Atlantic Ocean. In addition, a major dust event can be seen in the region of Pakistan and Afghanistan. For aerosol plumes at the most common height of 3 km, a TOMS aerosol index of less than 0.1 indicates a crystal clear sky with maximum visibility, whereas a value of 4 indicates the presence of aerosols so dense you would have difficulty seeing the mid-day sun. (The relationship between aerosol index and optical depth is dependent on altitude. Aerosols at low altitudes have a lower TOMS aerosol index than an equivalent depth of aerosol at a higher altitude).